Monitoring optical transmission lines and optical repeaters in the conventional optical amplification relay transmission line is known. Conventionally, a monitor circuit for monitoring the optical input and output levels and the like, is provided in each optical repeater. In accordance with one approach, a monitor command is sent from the terminal station to each optical repeater through the optical transmission line. When the optical repeater receives the command, the optical repeater returns onto the optical transmission line monitor information to the terminal station such as optical input and output levels. The transmission of signals such as the monitor command and the monitor information, is carried out by amplitude modulation of the main signal. This approach is shown in FIG. 10.
Another approach is disclosed in the unexamined but laid-open Japanese patent publication (Kokai) No. 1-1174339. In that approach an electric signal (burst-like sinusoidal wave) of a predetermined frequency is transmitted from the terminal station through a power feed line provided parallel to the optical transmission line. Each repeater filters an electric signal of the frequency, generates an electric signal of a frequency equal to n times the frequency of the above filtered electric signal, and transmits through the power feed line back to the terminal station. Thus, the electric signal which has the frequency equal to n times that of the original electric signal, and returned from each repeater, is filtered by the terminal station to be displayed on a CRT or the like. The signal returned by each repeater in response to the burst-like electric signal transmitted from the terminal station, is delayed depending on the distance from the terminal station to each relay, which determines when the signal reaches the terminal station. Therefore, the normal operation of the power feed line of the optical cable to each repeater can be monitored on the CRT display by displaying received levels of the returned signals scaled on the ordinate axis, and with the received times scaled on the abscissa axis.
According to the above-mentioned first approach, the control operation for the monitoring is carried out for each optical repeater. Therefore, it takes a long time to complete the monitoring of all of the optical repeaters, and a computer and complex software are necessary for performing the monitoring automatically.
According to the above-mentioned second approach, the normal operation of the optical signal transmission paths including optical fibers and optical repeaters are per se not monitored. However the normal operation of the power feed lines provided parallel to the optical transmission lines in the optical cables, are monitored. Therefore, the normal operation of the optical transmission lines can only be inferred based on the normal operation of the parallel-provided power feed lines. In particular, troubles such as increase in loss in fiber, break of fiber, degradation of characteristics of optical amplification repeaters, and the like, cannot be monitored. Further, electric signals must be modulated in each repeater. Therefore, the circuit constructions of the repeaters become more complex.
Accordingly, an object of the present invention is to provide an optical amplification relay system wherein the optical transmission lines and optical repeaters can be monitored in a short time, and the construction of the optical repeaters is simple.